Microorganisms (e. g., fungus, bacteria) have developed antibiotics to
compete for food sources. Antibiotic production appears to be important to
the survival of microorganisms through elimination of microbial competition
for food sources, which are usually very limited in soil. Different types
of antibiotics are made because these antibiotics have derived from
accumulation of mutations in their genetic material and have evolved upon
the microorganism they had to compete with. This evolution has led to
different kinds of antibiotics with different anti-microbial targets and
different specificities varying species to species and even from one strain
to another. Antibiotic production is very common among soil-dwelling
bacteria and fungi, and many of our most widely used medical antibiotics
(e.g., streptomycin) are made by soil microorganisms. If they can make
something that removes or drastically "slows-down" the competition for the
available nutrients, then they'll have an advantage.